The most widespread landing gear configuration in jet aircrafts is the “tricycle” configuration, which consists of two sets of main landing gear (port and starboard), and one set of forward or nose landing gear. The main landing gear is fitted to the aircraft wing, and the nose landing gear is fitted to a forward fuselage section.
The main role of the main landing gear is to withstand the main loads transmitted between the ground and the aircraft during landing, whereas the main role of the nose landing gear is to provide an additional point of support for the aircraft, and to steer the aircraft on the ground during taxing.
Conventionally, aircrafts are equipped with a hydraulic system to operate the landing gear, such as to extend or lower the landing gears for landing operations, and to retract the landing gear into corresponding gear bays for flight.
Modern aircrafts also include gear doors for the main and nose landing gears, which closes the landing gear bays during flight and during part of the landing operation. When an aircraft is rolling for taking off, the gear doors are normally closed and remain in this position after liftoff. As soon as the aircraft reaches a positive rate of climb, the pilot activates the landing gear retraction, and then the landing gear doors are open for receiving the landing gear into the landing gear bays, and after that, the gear doors are closed for the flight.
The sequence traditionally used in prior art aircrafts for extending the landing gear for landings is illustrated in FIGS. 1A and 1B, whereas the landing gear retraction for take-offs is illustrated in FIGS. 3A and 3B. Landing gear extension begins with the opening of the gear doors to expose the gear bays. Once the gear doors reach their final extended position, the extension of the nose and main landing gears is started, such that both the nose and main lading gears are lowered at the same time, while the gear doors remain open. Once nose and main landing gears reach their final extended position, the gear doors are closed to cover the landing gear bays.
First, in the retraction sequence (FIGS. 3A and 3B), the nose and main landing gear doors are open, and after reaching their fully open position, nose and main landing gears are retracted substantially at the same time. Once nose and main landing gears reach their final retracted position within the respective landing gear bays, nose and main landing gear doors are closed at the same time.
It can be observed in FIGS. 1A, 1B, 3A, and 3B that during most of the extension/retraction operations, the nose and main landing gear doors are extended while the nose and main landing gear are being extended/retracted.
It has been demonstrated that in that condition, the main cause of vibrations of the main landing gear doors is the aerodynamic excitation generated by the nose landing gear wake, impacting on the main landing gear doors.